I understand the workings of steam and hot water radiators, however there has been a nagging question in my mind about why we first heat water (for steam or hot water) before we heat the radiator. Why can't we use hot air to heat the radiators? For example, why can't I hold a propane torch at the bottom of an inlet opening of a radiator and heat the radiator that way? It seems to me that the radiator would heat faster and more efficiently that way.
Heat a radiator directly
radiator
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Best Answer
Heat transfer from water to the metal of the radiator is much more efficient than the heat transfer from the radiator metal to the air. Or in the case you posed from hot air to the radiator metal.
It is also relatively simple and compact to pipe the hot water or steam from room to room and floor to floor to service a series of radiators. A duct work to move hot air in the same way would be a lot larger in cross sectional area to transfer the same amount of heat energy from the heater to the radiators.